The invention relates generally to inorganic coating films, and more particularly to an improved inorganic coating film for optical materials.
Inorganic coating films are prepared by various methods including vacuum vapor deposition, ion plating, sputtering and the like. Such films are widely used as anti-reflection films, hard coats and other functional coatings of optical materials such as lenses and display device panels. Silicon dioxide is used extensively as an inorganic coating film because it readily adheres to most base materials and is hard and each to handle.
One of the critical defects of prior art inorganic coating films occurs when a water drop appears on the surface of the coating and dries. A water spot appears on the inorganic film which is similar to a spot on optical glass. The water spot affects the appearance and optical properties of the coating. As used herein, the term "spot" refers to stain which appears on optical glass and the term "water spot" refers to stains that appear on inorganic coating films. In the case of ophthalmic lenses, water drops often form on the lens surface during the edging process and during use. If the water drops are left on the lens surface for even a short period of time, a water spot appears.
In general, spots on optical glass are the result of the following processes:
(1) Ions in the glass including Na.sup.+, Ba.sup.2+, Pb.sup.2+ and the like are dissolved in the water drop and the water drop becomes an alkaline solution; and
(2) The alkaline water drop solution reacts with acidic gasses in the atmosphere, such as CO.sub.2 and SO.sub.2 to form compounds of Na.sub.2 CO.sub.3, NaHCO.sub.3, Na.sub.2 SO.sub.4, BaCO.sub.3, PbCO.sub.3, BaSO.sub.4, PbSO.sub.4 and the like. These compounds remain on the surface of the optical glass and cause the spot.
In contrast, inorganic coating films of SiO.sub.2 formed on synthetic resin bases do not contain positive ions and the reason for the formation of water spots had not been known until recently. It has now been determined that water spots are caused by an evaporation residue primarily of silicon compounds. These are produced in the water drop on the surface of the coating and strongly adhere to the surface. This phenomenon was determined as follows:
(1) When ultra pure water drops are placed onto the surface of a coating film, water spots did not appear. In the case of optical glass, even ultra pure water causes spots;
(2) When a water spot did appear and a section was taken through a portion of the coating surface where the water spot had appeared and was observed by transmissive electronic microscope (TEM), deposits were seen on the surface; and
(3) The elements of the deposits were analyzed and silicon was primarily detected.
Accordingly, it is desirable to provide an inorganic coating film that does not form water spots as a result of water droplets on the coating film.